As semiconductor integrated circuits are designed in a fine structure and/or in a highly integrated manner, a light source for exposure tends to have an even shorter wavelength. As a next generation light source for exposure of semiconductor, an extreme ultraviolet (EUV) light source is studied. Such light source can emit extreme ultraviolet light at a particular wavelength (i.e., 13.5 nm).
There are some known methods for the EUV light source device to generate (emit) the extreme ultraviolet light. One of the known methods heats an EUV radiation species (seed) for excitation. This generates a high temperature plasma. Then, the extreme ultraviolet light is extracted from the high temperature plasma.
The EUV light source device that employs such method is generally categorized into two types depending upon a way of generating the high temperature plasma. One type is a laser produced plasma (LPP) type EUV light source device. Another type is a discharge produced plasma (DPP) type EUV light source device.
A DPP type EUV light source device will be described briefly.
FIG. 10 of the accompanying drawing is a view useful to briefly describe a DPP type EUV light source device disclosed in Patent Literature 1. FIG. 11 illustrates a discharge electrode and a container in a D-D cross-section of FIG. 10. FIG. 12 is a set of cross-sectional views, each taken along the line A-A in FIG. 10.
The EUV light source device has a chamber 1, which is a discharge vessel. In the chamber 1, there are provided a discharge part 1a and an EUV light condensing part 1b. The discharge part 1a includes a pair of disc-shaped discharge electrodes 2a and 2b. The EUV light condensing part 1b includes a foil trap 5 and an EUV light condensing mirror 6, which is a light condensing unit.
A gas discharge unit 1c is used to evacuate the discharge part 1a and the EUV light condensing part 1b such that the interior of the chamber 1 becomes vacuum.
Reference numerals 2a and 2b designate disc-shaped discharge electrodes. The discharge electrodes 2a and 2b are spaced from each other by a predetermined distance. As motors 16a and 16b rotate, the electrodes 2a and 2b rotate about shafts 16c and 16d. 
A high temperature plasma material 14 is a material to emit EUV light at a wavelength of 13.5 nm. The plasma material 14 is, for example, liquid tin (Sn) and received in containers 15 and 15. The plasma material 14 is heated and becomes melted metal. As shown in FIG. 11, the temperature of the melted metal is adjusted by a temperature adjusting unit 15a disposed in, for example, each of the containers.
The electrodes 2a and 2b are partially immersed in the plasma material 14 in the associated containers 15 and 15, respectively. The liquid plasma material 14 that rides on the surface of each of the discharge electrodes 2a, 2b is conveyed into the discharge space upon rotation of the discharge electrode 2a, 2b. The high temperature plasma material 14 which is moved into the discharge space is irradiated with the laser beam 17 emitted from a laser source 17a. Upon irradiation with the laser beam 17, the high temperature plasma material 14 evaporates.
As shown in FIG. 11, for example, the laser beam is directed to the curved surface of the disc-shaped electrode 2a, 2b. 
As described above, each of the disc-shaped discharge electrodes is partly immersed in the associated container 15, and rotates. The container 15 retains the high temperature plasma material. Thus, as shown in FIG. 11, the high temperature plasma material, which is melted and received in the container, annularly adheres to the circular flat surface of the disc-shaped discharge electrode 2a, 2b. The high temperature plasma material also adheres to the curved surface of the disc-shaped discharge electrode 2a, 2b. 
As such, when the curved surface of the disc-shaped discharge electrode 2a, 2b is irradiated with the laser beam, the curved surface to which the high temperature plasma material adheres is an “area necessary for plasma” whereas the annular area on the circular flat surface to which the high temperature plasma material annularly adheres is an “area unnecessary for plasma.”
While the high temperature plasma material 14 is vaporized upon irradiation with the laser beam 17, a pulse electric power is applied to the electrodes 2a and 2b from a power source unit 4. Thus, a pulse discharge is triggered between the discharge electrodes 2a and 2b, and a plasma P is produced from the high temperature plasma material 14. A large current is caused to flow upon the discharging. The large current heats and excites the plasma such that the plasma temperature is elevated. As a result, the EUV light is emitted from the high temperature plasma P.
It should be noted that the pulse electric power is applied between the discharge electrodes 2a and 2b. Thus, the resulting discharge is the pulse discharge, and the emitted EUV light is light emitted like a pulse, i.e., pulse light (pulsing light).
The EUV light emitted from the high temperature plasma P is condensed to a condensing point f of the light condensing mirror 6 (also referred to as “intermediate condensing point f” in this specification) by the EUV light condensing mirror 6. Then, the EUV light exits from an EUV light outlet 7, and is incident to an exposure equipment 40 attached to the EUV light source device. The exposure equipment 40 is indicated by the broken line.
According to this method, it is easy to vaporize Sn, which is solid at room temperature, in the vicinity of the discharge region where the discharge takes place. The discharge region is the space for the discharge between the discharge electrodes. Specifically, it is possible to efficiently feed the vaporized Sn to the discharge region, and therefore it becomes possible to efficiently extract the EUV radiation at the wavelength of 13.5 nm after the discharging.
The EUV light source device disclosed in Patent Literature 1 has the following advantages because the discharge electrodes are caused to rotate.
(1) It is possible to always feed a solid or liquid high temperature plasma material to the discharge region. The plasma material is a fresh material of an EUV generation species.
(2) Because the position on each discharge electrode surface, which is irradiated with the laser beam, and the position of the high temperature plasma generation (position of the discharge part) always change, the thermal load on each discharge electrode reduces, and therefore it is possible to reduce or prevent the wear of the discharge electrodes.